xref: /freebsd/contrib/llvm-project/libcxx/include/__expected/expected.h (revision 38c63bdc46252d4d8cd313dff4183ec4546d26d9) 
1 // -*- C++ -*-
2 //===----------------------------------------------------------------------===//
3 //
4 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
5 // See https://llvm.org/LICENSE.txt for license information.
6 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
7 //
8 //===----------------------------------------------------------------------===//
9 #ifndef _LIBCPP___EXPECTED_EXPECTED_H
10 #define _LIBCPP___EXPECTED_EXPECTED_H
11 
12 #include <__assert>
13 #include <__config>
14 #include <__expected/bad_expected_access.h>
15 #include <__expected/unexpect.h>
16 #include <__expected/unexpected.h>
17 #include <__functional/invoke.h>
18 #include <__memory/addressof.h>
19 #include <__memory/construct_at.h>
20 #include <__type_traits/conjunction.h>
21 #include <__type_traits/disjunction.h>
22 #include <__type_traits/integral_constant.h>
23 #include <__type_traits/is_assignable.h>
24 #include <__type_traits/is_constructible.h>
25 #include <__type_traits/is_convertible.h>
26 #include <__type_traits/is_copy_assignable.h>
27 #include <__type_traits/is_copy_constructible.h>
28 #include <__type_traits/is_default_constructible.h>
29 #include <__type_traits/is_function.h>
30 #include <__type_traits/is_move_assignable.h>
31 #include <__type_traits/is_move_constructible.h>
32 #include <__type_traits/is_nothrow_constructible.h>
33 #include <__type_traits/is_nothrow_copy_assignable.h>
34 #include <__type_traits/is_nothrow_copy_constructible.h>
35 #include <__type_traits/is_nothrow_default_constructible.h>
36 #include <__type_traits/is_nothrow_move_assignable.h>
37 #include <__type_traits/is_nothrow_move_constructible.h>
38 #include <__type_traits/is_reference.h>
39 #include <__type_traits/is_same.h>
40 #include <__type_traits/is_swappable.h>
41 #include <__type_traits/is_trivially_copy_constructible.h>
42 #include <__type_traits/is_trivially_destructible.h>
43 #include <__type_traits/is_trivially_move_constructible.h>
44 #include <__type_traits/is_void.h>
45 #include <__type_traits/lazy.h>
46 #include <__type_traits/negation.h>
47 #include <__type_traits/remove_cv.h>
48 #include <__type_traits/remove_cvref.h>
49 #include <__utility/as_const.h>
50 #include <__utility/exception_guard.h>
51 #include <__utility/forward.h>
52 #include <__utility/in_place.h>
53 #include <__utility/move.h>
54 #include <__utility/swap.h>
55 #include <__verbose_abort>
56 #include <initializer_list>
57 
58 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
59 #  pragma GCC system_header
60 #endif
61 
62 _LIBCPP_PUSH_MACROS
63 #include <__undef_macros>
64 
65 #if _LIBCPP_STD_VER >= 23
66 
67 _LIBCPP_BEGIN_NAMESPACE_STD
68 
69 template <class _Tp, class _Err>
70 class expected;
71 
72 template <class _Tp>
73 struct __is_std_expected : false_type {};
74 
75 template <class _Tp, class _Err>
76 struct __is_std_expected<expected<_Tp, _Err>> : true_type {};
77 
78 struct __expected_construct_in_place_from_invoke_tag {};
79 struct __expected_construct_unexpected_from_invoke_tag {};
80 
81 template <class _Err, class _Arg>
82 _LIBCPP_HIDE_FROM_ABI void __throw_bad_expected_access(_Arg&& __arg) {
83 #  ifndef _LIBCPP_HAS_NO_EXCEPTIONS
84   throw bad_expected_access<_Err>(std::forward<_Arg>(__arg));
85 #  else
86   (void)__arg;
87   _LIBCPP_VERBOSE_ABORT("bad_expected_access was thrown in -fno-exceptions mode");
88 #  endif
89 }
90 
91 template <class _Tp, class _Err>
92 class expected {
93   static_assert(
94       !is_reference_v<_Tp> &&
95           !is_function_v<_Tp> &&
96           !is_same_v<remove_cv_t<_Tp>, in_place_t> &&
97           !is_same_v<remove_cv_t<_Tp>, unexpect_t> &&
98           !__is_std_unexpected<remove_cv_t<_Tp>>::value &&
99           __valid_std_unexpected<_Err>::value
100       ,
101       "[expected.object.general] A program that instantiates the definition of template expected<T, E> for a "
102       "reference type, a function type, or for possibly cv-qualified types in_place_t, unexpect_t, or a "
103       "specialization of unexpected for the T parameter is ill-formed. A program that instantiates the "
104       "definition of the template expected<T, E> with a type for the E parameter that is not a valid "
105       "template argument for unexpected is ill-formed.");
106 
107   template <class _Up, class _OtherErr>
108   friend class expected;
109 
110 public:
111   using value_type      = _Tp;
112   using error_type      = _Err;
113   using unexpected_type = unexpected<_Err>;
114 
115   template <class _Up>
116   using rebind = expected<_Up, error_type>;
117 
118   // [expected.object.ctor], constructors
119   _LIBCPP_HIDE_FROM_ABI constexpr expected()
120     noexcept(is_nothrow_default_constructible_v<_Tp>) // strengthened
121     requires is_default_constructible_v<_Tp>
122       : __union_(std::in_place), __has_val_(true) {}
123 
124   _LIBCPP_HIDE_FROM_ABI constexpr expected(const expected&) = delete;
125 
126   _LIBCPP_HIDE_FROM_ABI constexpr expected(const expected&)
127     requires(is_copy_constructible_v<_Tp> &&
128              is_copy_constructible_v<_Err> &&
129              is_trivially_copy_constructible_v<_Tp> &&
130              is_trivially_copy_constructible_v<_Err>)
131   = default;
132 
133   _LIBCPP_HIDE_FROM_ABI constexpr expected(const expected& __other)
134     noexcept(is_nothrow_copy_constructible_v<_Tp> && is_nothrow_copy_constructible_v<_Err>) // strengthened
135     requires(is_copy_constructible_v<_Tp> && is_copy_constructible_v<_Err> &&
136              !(is_trivially_copy_constructible_v<_Tp> && is_trivially_copy_constructible_v<_Err>))
137       : __union_(__other.__has_val_, __other.__union_), __has_val_(__other.__has_val_) { }
138 
139   _LIBCPP_HIDE_FROM_ABI constexpr expected(expected&&)
140     requires(is_move_constructible_v<_Tp> && is_move_constructible_v<_Err>
141               && is_trivially_move_constructible_v<_Tp> && is_trivially_move_constructible_v<_Err>)
142   = default;
143 
144   _LIBCPP_HIDE_FROM_ABI constexpr expected(expected&& __other)
145     noexcept(is_nothrow_move_constructible_v<_Tp> && is_nothrow_move_constructible_v<_Err>)
146     requires(is_move_constructible_v<_Tp> && is_move_constructible_v<_Err> &&
147              !(is_trivially_move_constructible_v<_Tp> && is_trivially_move_constructible_v<_Err>))
148       : __union_(__other.__has_val_, std::move(__other.__union_)), __has_val_(__other.__has_val_) { }
149 
150 private:
151   template <class _Up, class _OtherErr, class _UfQual, class _OtherErrQual>
152   using __can_convert =
153       _And< is_constructible<_Tp, _UfQual>,
154             is_constructible<_Err, _OtherErrQual>,
155             _Not<is_constructible<_Tp, expected<_Up, _OtherErr>&>>,
156             _Not<is_constructible<_Tp, expected<_Up, _OtherErr>>>,
157             _Not<is_constructible<_Tp, const expected<_Up, _OtherErr>&>>,
158             _Not<is_constructible<_Tp, const expected<_Up, _OtherErr>>>,
159             _Not<is_convertible<expected<_Up, _OtherErr>&, _Tp>>,
160             _Not<is_convertible<expected<_Up, _OtherErr>&&, _Tp>>,
161             _Not<is_convertible<const expected<_Up, _OtherErr>&, _Tp>>,
162             _Not<is_convertible<const expected<_Up, _OtherErr>&&, _Tp>>,
163             _Not<is_constructible<unexpected<_Err>, expected<_Up, _OtherErr>&>>,
164             _Not<is_constructible<unexpected<_Err>, expected<_Up, _OtherErr>>>,
165             _Not<is_constructible<unexpected<_Err>, const expected<_Up, _OtherErr>&>>,
166             _Not<is_constructible<unexpected<_Err>, const expected<_Up, _OtherErr>>> >;
167 
168   template <class _Func, class... _Args>
169   _LIBCPP_HIDE_FROM_ABI constexpr explicit expected(
170       std::__expected_construct_in_place_from_invoke_tag __tag, _Func&& __f, _Args&&... __args)
171       : __union_(__tag, std::forward<_Func>(__f), std::forward<_Args>(__args)...), __has_val_(true) {}
172 
173   template <class _Func, class... _Args>
174   _LIBCPP_HIDE_FROM_ABI constexpr explicit expected(
175       std::__expected_construct_unexpected_from_invoke_tag __tag, _Func&& __f, _Args&&... __args)
176       : __union_(__tag, std::forward<_Func>(__f), std::forward<_Args>(__args)...), __has_val_(false) {}
177 
178 public:
179   template <class _Up, class _OtherErr>
180     requires __can_convert<_Up, _OtherErr, const _Up&, const _OtherErr&>::value
181   _LIBCPP_HIDE_FROM_ABI constexpr explicit(!is_convertible_v<const _Up&, _Tp> ||
182                                            !is_convertible_v<const _OtherErr&, _Err>)
183   expected(const expected<_Up, _OtherErr>& __other)
184     noexcept(is_nothrow_constructible_v<_Tp, const _Up&> &&
185              is_nothrow_constructible_v<_Err, const _OtherErr&>) // strengthened
186       : __union_(__other.__has_val_, __other.__union_), __has_val_(__other.__has_val_) {}
187 
188   template <class _Up, class _OtherErr>
189     requires __can_convert<_Up, _OtherErr, _Up, _OtherErr>::value
190   _LIBCPP_HIDE_FROM_ABI constexpr explicit(!is_convertible_v<_Up, _Tp> || !is_convertible_v<_OtherErr, _Err>)
191   expected(expected<_Up, _OtherErr>&& __other)
192     noexcept(is_nothrow_constructible_v<_Tp, _Up> && is_nothrow_constructible_v<_Err, _OtherErr>) // strengthened
193       : __union_(__other.__has_val_, std::move(__other.__union_)), __has_val_(__other.__has_val_) {}
194 
195   template <class _Up = _Tp>
196     requires(!is_same_v<remove_cvref_t<_Up>, in_place_t> && !is_same_v<expected, remove_cvref_t<_Up>> &&
197              !__is_std_unexpected<remove_cvref_t<_Up>>::value && is_constructible_v<_Tp, _Up>)
198   _LIBCPP_HIDE_FROM_ABI constexpr explicit(!is_convertible_v<_Up, _Tp>)
199       expected(_Up&& __u) noexcept(is_nothrow_constructible_v<_Tp, _Up>) // strengthened
200       : __union_(std::in_place, std::forward<_Up>(__u)), __has_val_(true) {}
201 
202 
203   template <class _OtherErr>
204     requires is_constructible_v<_Err, const _OtherErr&>
205   _LIBCPP_HIDE_FROM_ABI constexpr explicit(!is_convertible_v<const _OtherErr&, _Err>)
206   expected(const unexpected<_OtherErr>& __unex)
207     noexcept(is_nothrow_constructible_v<_Err, const _OtherErr&>) // strengthened
208       : __union_(std::unexpect, __unex.error()), __has_val_(false) {}
209 
210   template <class _OtherErr>
211     requires is_constructible_v<_Err, _OtherErr>
212   _LIBCPP_HIDE_FROM_ABI constexpr explicit(!is_convertible_v<_OtherErr, _Err>)
213   expected(unexpected<_OtherErr>&& __unex)
214     noexcept(is_nothrow_constructible_v<_Err, _OtherErr>) // strengthened
215       : __union_(std::unexpect, std::move(__unex.error())), __has_val_(false) {}
216 
217   template <class... _Args>
218     requires is_constructible_v<_Tp, _Args...>
219   _LIBCPP_HIDE_FROM_ABI constexpr explicit expected(in_place_t, _Args&&... __args)
220     noexcept(is_nothrow_constructible_v<_Tp, _Args...>) // strengthened
221       : __union_(std::in_place, std::forward<_Args>(__args)...), __has_val_(true) {}
222 
223   template <class _Up, class... _Args>
224     requires is_constructible_v< _Tp, initializer_list<_Up>&, _Args... >
225   _LIBCPP_HIDE_FROM_ABI constexpr explicit
226   expected(in_place_t, initializer_list<_Up> __il, _Args&&... __args)
227     noexcept(is_nothrow_constructible_v<_Tp, initializer_list<_Up>&, _Args...>) // strengthened
228       : __union_(std::in_place, __il, std::forward<_Args>(__args)...), __has_val_(true) {}
229 
230   template <class... _Args>
231     requires is_constructible_v<_Err, _Args...>
232   _LIBCPP_HIDE_FROM_ABI constexpr explicit expected(unexpect_t, _Args&&... __args)
233     noexcept(is_nothrow_constructible_v<_Err, _Args...>) // strengthened
234       : __union_(std::unexpect, std::forward<_Args>(__args)...), __has_val_(false) {}
235 
236   template <class _Up, class... _Args>
237     requires is_constructible_v< _Err, initializer_list<_Up>&, _Args... >
238   _LIBCPP_HIDE_FROM_ABI constexpr explicit
239   expected(unexpect_t, initializer_list<_Up> __il, _Args&&... __args)
240     noexcept(is_nothrow_constructible_v<_Err, initializer_list<_Up>&, _Args...>) // strengthened
241       : __union_(std::unexpect, __il, std::forward<_Args>(__args)...), __has_val_(false) {}
242 
243   // [expected.object.dtor], destructor
244 
245   _LIBCPP_HIDE_FROM_ABI constexpr ~expected()
246     requires(is_trivially_destructible_v<_Tp> && is_trivially_destructible_v<_Err>)
247   = default;
248 
249   _LIBCPP_HIDE_FROM_ABI constexpr ~expected()
250     requires(!is_trivially_destructible_v<_Tp> || !is_trivially_destructible_v<_Err>)
251   {
252     if (__has_val_) {
253       std::destroy_at(std::addressof(__union_.__val_));
254     } else {
255       std::destroy_at(std::addressof(__union_.__unex_));
256     }
257   }
258 
259 private:
260   template <class _T1, class _T2, class... _Args>
261   _LIBCPP_HIDE_FROM_ABI static constexpr void __reinit_expected(_T1& __newval, _T2& __oldval, _Args&&... __args) {
262     if constexpr (is_nothrow_constructible_v<_T1, _Args...>) {
263       std::destroy_at(std::addressof(__oldval));
264       std::construct_at(std::addressof(__newval), std::forward<_Args>(__args)...);
265     } else if constexpr (is_nothrow_move_constructible_v<_T1>) {
266       _T1 __tmp(std::forward<_Args>(__args)...);
267       std::destroy_at(std::addressof(__oldval));
268       std::construct_at(std::addressof(__newval), std::move(__tmp));
269     } else {
270       static_assert(
271           is_nothrow_move_constructible_v<_T2>,
272           "To provide strong exception guarantee, T2 has to satisfy `is_nothrow_move_constructible_v` so that it can "
273           "be reverted to the previous state in case an exception is thrown during the assignment.");
274       _T2 __tmp(std::move(__oldval));
275       std::destroy_at(std::addressof(__oldval));
276       auto __trans =
277           std::__make_exception_guard([&] { std::construct_at(std::addressof(__oldval), std::move(__tmp)); });
278       std::construct_at(std::addressof(__newval), std::forward<_Args>(__args)...);
279       __trans.__complete();
280     }
281   }
282 
283 public:
284   // [expected.object.assign], assignment
285   _LIBCPP_HIDE_FROM_ABI constexpr expected& operator=(const expected&) = delete;
286 
287   _LIBCPP_HIDE_FROM_ABI constexpr expected& operator=(const expected& __rhs)
288     noexcept(is_nothrow_copy_assignable_v<_Tp> &&
289              is_nothrow_copy_constructible_v<_Tp> &&
290              is_nothrow_copy_assignable_v<_Err> &&
291              is_nothrow_copy_constructible_v<_Err>) // strengthened
292     requires(is_copy_assignable_v<_Tp> &&
293              is_copy_constructible_v<_Tp> &&
294              is_copy_assignable_v<_Err> &&
295              is_copy_constructible_v<_Err> &&
296              (is_nothrow_move_constructible_v<_Tp> ||
297               is_nothrow_move_constructible_v<_Err>))
298   {
299     if (__has_val_ && __rhs.__has_val_) {
300       __union_.__val_ = __rhs.__union_.__val_;
301     } else if (__has_val_) {
302       __reinit_expected(__union_.__unex_, __union_.__val_, __rhs.__union_.__unex_);
303     } else if (__rhs.__has_val_) {
304       __reinit_expected(__union_.__val_, __union_.__unex_, __rhs.__union_.__val_);
305     } else {
306       __union_.__unex_ = __rhs.__union_.__unex_;
307     }
308     // note: only reached if no exception+rollback was done inside __reinit_expected
309     __has_val_ = __rhs.__has_val_;
310     return *this;
311   }
312 
313   _LIBCPP_HIDE_FROM_ABI constexpr expected& operator=(expected&& __rhs)
314     noexcept(is_nothrow_move_assignable_v<_Tp> &&
315              is_nothrow_move_constructible_v<_Tp> &&
316              is_nothrow_move_assignable_v<_Err> &&
317              is_nothrow_move_constructible_v<_Err>)
318     requires(is_move_constructible_v<_Tp> &&
319              is_move_assignable_v<_Tp> &&
320              is_move_constructible_v<_Err> &&
321              is_move_assignable_v<_Err> &&
322              (is_nothrow_move_constructible_v<_Tp> ||
323               is_nothrow_move_constructible_v<_Err>))
324   {
325     if (__has_val_ && __rhs.__has_val_) {
326       __union_.__val_ = std::move(__rhs.__union_.__val_);
327     } else if (__has_val_) {
328       __reinit_expected(__union_.__unex_, __union_.__val_, std::move(__rhs.__union_.__unex_));
329     } else if (__rhs.__has_val_) {
330       __reinit_expected(__union_.__val_, __union_.__unex_, std::move(__rhs.__union_.__val_));
331     } else {
332       __union_.__unex_ = std::move(__rhs.__union_.__unex_);
333     }
334     // note: only reached if no exception+rollback was done inside __reinit_expected
335     __has_val_ = __rhs.__has_val_;
336     return *this;
337   }
338 
339   template <class _Up = _Tp>
340   _LIBCPP_HIDE_FROM_ABI constexpr expected& operator=(_Up&& __v)
341     requires(!is_same_v<expected, remove_cvref_t<_Up>> &&
342              !__is_std_unexpected<remove_cvref_t<_Up>>::value &&
343              is_constructible_v<_Tp, _Up> &&
344              is_assignable_v<_Tp&, _Up> &&
345              (is_nothrow_constructible_v<_Tp, _Up> ||
346               is_nothrow_move_constructible_v<_Tp> ||
347               is_nothrow_move_constructible_v<_Err>))
348   {
349     if (__has_val_) {
350       __union_.__val_ = std::forward<_Up>(__v);
351     } else {
352       __reinit_expected(__union_.__val_, __union_.__unex_, std::forward<_Up>(__v));
353       __has_val_ = true;
354     }
355     return *this;
356   }
357 
358 private:
359   template <class _OtherErrQual>
360   static constexpr bool __can_assign_from_unexpected =
361       _And< is_constructible<_Err, _OtherErrQual>,
362             is_assignable<_Err&, _OtherErrQual>,
363             _Lazy<_Or,
364                   is_nothrow_constructible<_Err, _OtherErrQual>,
365                   is_nothrow_move_constructible<_Tp>,
366                   is_nothrow_move_constructible<_Err>> >::value;
367 
368 public:
369   template <class _OtherErr>
370     requires(__can_assign_from_unexpected<const _OtherErr&>)
371   _LIBCPP_HIDE_FROM_ABI constexpr expected& operator=(const unexpected<_OtherErr>& __un) {
372     if (__has_val_) {
373       __reinit_expected(__union_.__unex_, __union_.__val_, __un.error());
374       __has_val_ = false;
375     } else {
376       __union_.__unex_ = __un.error();
377     }
378     return *this;
379   }
380 
381   template <class _OtherErr>
382     requires(__can_assign_from_unexpected<_OtherErr>)
383   _LIBCPP_HIDE_FROM_ABI constexpr expected& operator=(unexpected<_OtherErr>&& __un) {
384     if (__has_val_) {
385       __reinit_expected(__union_.__unex_, __union_.__val_, std::move(__un.error()));
386       __has_val_ = false;
387     } else {
388       __union_.__unex_ = std::move(__un.error());
389     }
390     return *this;
391   }
392 
393   template <class... _Args>
394     requires is_nothrow_constructible_v<_Tp, _Args...>
395   _LIBCPP_HIDE_FROM_ABI constexpr _Tp& emplace(_Args&&... __args) noexcept {
396     if (__has_val_) {
397       std::destroy_at(std::addressof(__union_.__val_));
398     } else {
399       std::destroy_at(std::addressof(__union_.__unex_));
400     }
401     std::construct_at(std::addressof(__union_.__val_), std::forward<_Args>(__args)...);
402     __has_val_ = true;
403     return __union_.__val_;
404   }
405 
406   template <class _Up, class... _Args>
407     requires is_nothrow_constructible_v< _Tp, initializer_list<_Up>&, _Args... >
408   _LIBCPP_HIDE_FROM_ABI constexpr _Tp& emplace(initializer_list<_Up> __il, _Args&&... __args) noexcept {
409     if (__has_val_) {
410       std::destroy_at(std::addressof(__union_.__val_));
411     } else {
412       std::destroy_at(std::addressof(__union_.__unex_));
413     }
414     std::construct_at(std::addressof(__union_.__val_), __il, std::forward<_Args>(__args)...);
415     __has_val_ = true;
416     return __union_.__val_;
417   }
418 
419 
420 public:
421   // [expected.object.swap], swap
422   _LIBCPP_HIDE_FROM_ABI constexpr void swap(expected& __rhs)
423     noexcept(is_nothrow_move_constructible_v<_Tp> &&
424              is_nothrow_swappable_v<_Tp> &&
425              is_nothrow_move_constructible_v<_Err> &&
426              is_nothrow_swappable_v<_Err>)
427     requires(is_swappable_v<_Tp> &&
428              is_swappable_v<_Err> &&
429              is_move_constructible_v<_Tp> &&
430              is_move_constructible_v<_Err> &&
431              (is_nothrow_move_constructible_v<_Tp> ||
432               is_nothrow_move_constructible_v<_Err>))
433   {
434     auto __swap_val_unex_impl = [&](expected& __with_val, expected& __with_err) {
435       if constexpr (is_nothrow_move_constructible_v<_Err>) {
436         _Err __tmp(std::move(__with_err.__union_.__unex_));
437         std::destroy_at(std::addressof(__with_err.__union_.__unex_));
438         auto __trans = std::__make_exception_guard([&] {
439           std::construct_at(std::addressof(__with_err.__union_.__unex_), std::move(__tmp));
440         });
441         std::construct_at(std::addressof(__with_err.__union_.__val_), std::move(__with_val.__union_.__val_));
442         __trans.__complete();
443         std::destroy_at(std::addressof(__with_val.__union_.__val_));
444         std::construct_at(std::addressof(__with_val.__union_.__unex_), std::move(__tmp));
445       } else {
446         static_assert(is_nothrow_move_constructible_v<_Tp>,
447                       "To provide strong exception guarantee, Tp has to satisfy `is_nothrow_move_constructible_v` so "
448                       "that it can be reverted to the previous state in case an exception is thrown during swap.");
449         _Tp __tmp(std::move(__with_val.__union_.__val_));
450         std::destroy_at(std::addressof(__with_val.__union_.__val_));
451         auto __trans = std::__make_exception_guard([&] {
452           std::construct_at(std::addressof(__with_val.__union_.__val_), std::move(__tmp));
453         });
454         std::construct_at(std::addressof(__with_val.__union_.__unex_), std::move(__with_err.__union_.__unex_));
455         __trans.__complete();
456         std::destroy_at(std::addressof(__with_err.__union_.__unex_));
457         std::construct_at(std::addressof(__with_err.__union_.__val_), std::move(__tmp));
458       }
459       __with_val.__has_val_ = false;
460       __with_err.__has_val_ = true;
461     };
462 
463     if (__has_val_) {
464       if (__rhs.__has_val_) {
465         using std::swap;
466         swap(__union_.__val_, __rhs.__union_.__val_);
467       } else {
468         __swap_val_unex_impl(*this, __rhs);
469       }
470     } else {
471       if (__rhs.__has_val_) {
472         __swap_val_unex_impl(__rhs, *this);
473       } else {
474         using std::swap;
475         swap(__union_.__unex_, __rhs.__union_.__unex_);
476       }
477     }
478   }
479 
480   _LIBCPP_HIDE_FROM_ABI friend constexpr void swap(expected& __x, expected& __y)
481     noexcept(noexcept(__x.swap(__y)))
482     requires requires { __x.swap(__y); }
483   {
484     __x.swap(__y);
485   }
486 
487   // [expected.object.obs], observers
488   _LIBCPP_HIDE_FROM_ABI constexpr const _Tp* operator->() const noexcept {
489     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(__has_val_, "expected::operator-> requires the expected to contain a value");
490     return std::addressof(__union_.__val_);
491   }
492 
493   _LIBCPP_HIDE_FROM_ABI constexpr _Tp* operator->() noexcept {
494     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(__has_val_, "expected::operator-> requires the expected to contain a value");
495     return std::addressof(__union_.__val_);
496   }
497 
498   _LIBCPP_HIDE_FROM_ABI constexpr const _Tp& operator*() const& noexcept {
499     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(__has_val_, "expected::operator* requires the expected to contain a value");
500     return __union_.__val_;
501   }
502 
503   _LIBCPP_HIDE_FROM_ABI constexpr _Tp& operator*() & noexcept {
504     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(__has_val_, "expected::operator* requires the expected to contain a value");
505     return __union_.__val_;
506   }
507 
508   _LIBCPP_HIDE_FROM_ABI constexpr const _Tp&& operator*() const&& noexcept {
509     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(__has_val_, "expected::operator* requires the expected to contain a value");
510     return std::move(__union_.__val_);
511   }
512 
513   _LIBCPP_HIDE_FROM_ABI constexpr _Tp&& operator*() && noexcept {
514     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(__has_val_, "expected::operator* requires the expected to contain a value");
515     return std::move(__union_.__val_);
516   }
517 
518   _LIBCPP_HIDE_FROM_ABI constexpr explicit operator bool() const noexcept { return __has_val_; }
519 
520   _LIBCPP_HIDE_FROM_ABI constexpr bool has_value() const noexcept { return __has_val_; }
521 
522   _LIBCPP_HIDE_FROM_ABI constexpr const _Tp& value() const& {
523     static_assert(is_copy_constructible_v<_Err>, "error_type has to be copy constructible");
524     if (!__has_val_) {
525       std::__throw_bad_expected_access<_Err>(std::as_const(error()));
526     }
527     return __union_.__val_;
528   }
529 
530   _LIBCPP_HIDE_FROM_ABI constexpr _Tp& value() & {
531     static_assert(is_copy_constructible_v<_Err>, "error_type has to be copy constructible");
532     if (!__has_val_) {
533       std::__throw_bad_expected_access<_Err>(std::as_const(error()));
534     }
535     return __union_.__val_;
536   }
537 
538   _LIBCPP_HIDE_FROM_ABI constexpr const _Tp&& value() const&& {
539     static_assert(is_copy_constructible_v<_Err> && is_constructible_v<_Err, decltype(std::move(error()))>,
540                   "error_type has to be both copy constructible and constructible from decltype(std::move(error()))");
541     if (!__has_val_) {
542       std::__throw_bad_expected_access<_Err>(std::move(error()));
543     }
544     return std::move(__union_.__val_);
545   }
546 
547   _LIBCPP_HIDE_FROM_ABI constexpr _Tp&& value() && {
548     static_assert(is_copy_constructible_v<_Err> && is_constructible_v<_Err, decltype(std::move(error()))>,
549                   "error_type has to be both copy constructible and constructible from decltype(std::move(error()))");
550     if (!__has_val_) {
551       std::__throw_bad_expected_access<_Err>(std::move(error()));
552     }
553     return std::move(__union_.__val_);
554   }
555 
556   _LIBCPP_HIDE_FROM_ABI constexpr const _Err& error() const& noexcept {
557     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(!__has_val_, "expected::error requires the expected to contain an error");
558     return __union_.__unex_;
559   }
560 
561   _LIBCPP_HIDE_FROM_ABI constexpr _Err& error() & noexcept {
562     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(!__has_val_, "expected::error requires the expected to contain an error");
563     return __union_.__unex_;
564   }
565 
566   _LIBCPP_HIDE_FROM_ABI constexpr const _Err&& error() const&& noexcept {
567     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(!__has_val_, "expected::error requires the expected to contain an error");
568     return std::move(__union_.__unex_);
569   }
570 
571   _LIBCPP_HIDE_FROM_ABI constexpr _Err&& error() && noexcept {
572     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(!__has_val_, "expected::error requires the expected to contain an error");
573     return std::move(__union_.__unex_);
574   }
575 
576   template <class _Up>
577   _LIBCPP_HIDE_FROM_ABI constexpr _Tp value_or(_Up&& __v) const& {
578     static_assert(is_copy_constructible_v<_Tp>, "value_type has to be copy constructible");
579     static_assert(is_convertible_v<_Up, _Tp>, "argument has to be convertible to value_type");
580     return __has_val_ ? __union_.__val_ : static_cast<_Tp>(std::forward<_Up>(__v));
581   }
582 
583   template <class _Up>
584   _LIBCPP_HIDE_FROM_ABI constexpr _Tp value_or(_Up&& __v) && {
585     static_assert(is_move_constructible_v<_Tp>, "value_type has to be move constructible");
586     static_assert(is_convertible_v<_Up, _Tp>, "argument has to be convertible to value_type");
587     return __has_val_ ? std::move(__union_.__val_) : static_cast<_Tp>(std::forward<_Up>(__v));
588   }
589 
590   template <class _Up = _Err>
591   _LIBCPP_HIDE_FROM_ABI constexpr _Err error_or(_Up&& __error) const& {
592     static_assert(is_copy_constructible_v<_Err>, "error_type has to be copy constructible");
593     static_assert(is_convertible_v<_Up, _Err>, "argument has to be convertible to error_type");
594     if (has_value())
595       return std::forward<_Up>(__error);
596     return error();
597   }
598 
599   template <class _Up = _Err>
600   _LIBCPP_HIDE_FROM_ABI constexpr _Err error_or(_Up&& __error) && {
601     static_assert(is_move_constructible_v<_Err>, "error_type has to be move constructible");
602     static_assert(is_convertible_v<_Up, _Err>, "argument has to be convertible to error_type");
603     if (has_value())
604       return std::forward<_Up>(__error);
605     return std::move(error());
606   }
607 
608   // [expected.void.monadic], monadic
609   template <class _Func>
610     requires is_constructible_v<_Err, _Err&>
611   _LIBCPP_HIDE_FROM_ABI constexpr auto and_then(_Func&& __f) & {
612     using _Up = remove_cvref_t<invoke_result_t<_Func, _Tp&>>;
613     static_assert(__is_std_expected<_Up>::value, "The result of f(value()) must be a specialization of std::expected");
614     static_assert(is_same_v<typename _Up::error_type, _Err>,
615                   "The result of f(value()) must have the same error_type as this expected");
616     if (has_value()) {
617       return std::invoke(std::forward<_Func>(__f), value());
618     }
619     return _Up(unexpect, error());
620   }
621 
622   template <class _Func>
623     requires is_constructible_v<_Err, const _Err&>
624   _LIBCPP_HIDE_FROM_ABI constexpr auto and_then(_Func&& __f) const& {
625     using _Up = remove_cvref_t<invoke_result_t<_Func, const _Tp&>>;
626     static_assert(__is_std_expected<_Up>::value, "The result of f(value()) must be a specialization of std::expected");
627     static_assert(is_same_v<typename _Up::error_type, _Err>,
628                   "The result of f(value()) must have the same error_type as this expected");
629     if (has_value()) {
630       return std::invoke(std::forward<_Func>(__f), value());
631     }
632     return _Up(unexpect, error());
633   }
634 
635   template <class _Func>
636     requires is_constructible_v<_Err, _Err&&>
637   _LIBCPP_HIDE_FROM_ABI constexpr auto and_then(_Func&& __f) && {
638     using _Up = remove_cvref_t<invoke_result_t<_Func, _Tp&&>>;
639     static_assert(
640         __is_std_expected<_Up>::value, "The result of f(std::move(value())) must be a specialization of std::expected");
641     static_assert(is_same_v<typename _Up::error_type, _Err>,
642                   "The result of f(std::move(value())) must have the same error_type as this expected");
643     if (has_value()) {
644       return std::invoke(std::forward<_Func>(__f), std::move(value()));
645     }
646     return _Up(unexpect, std::move(error()));
647   }
648 
649   template <class _Func>
650     requires is_constructible_v<_Err, const _Err&&>
651   _LIBCPP_HIDE_FROM_ABI constexpr auto and_then(_Func&& __f) const&& {
652     using _Up = remove_cvref_t<invoke_result_t<_Func, const _Tp&&>>;
653     static_assert(
654         __is_std_expected<_Up>::value, "The result of f(std::move(value())) must be a specialization of std::expected");
655     static_assert(is_same_v<typename _Up::error_type, _Err>,
656                   "The result of f(std::move(value())) must have the same error_type as this expected");
657     if (has_value()) {
658       return std::invoke(std::forward<_Func>(__f), std::move(value()));
659     }
660     return _Up(unexpect, std::move(error()));
661   }
662 
663   template <class _Func>
664     requires is_constructible_v<_Tp, _Tp&>
665   _LIBCPP_HIDE_FROM_ABI constexpr auto or_else(_Func&& __f) & {
666     using _Gp = remove_cvref_t<invoke_result_t<_Func, _Err&>>;
667     static_assert(__is_std_expected<_Gp>::value, "The result of f(error()) must be a specialization of std::expected");
668     static_assert(is_same_v<typename _Gp::value_type, _Tp>,
669                   "The result of f(error()) must have the same value_type as this expected");
670     if (has_value()) {
671       return _Gp(in_place, value());
672     }
673     return std::invoke(std::forward<_Func>(__f), error());
674   }
675 
676   template <class _Func>
677     requires is_constructible_v<_Tp, const _Tp&>
678   _LIBCPP_HIDE_FROM_ABI constexpr auto or_else(_Func&& __f) const& {
679     using _Gp = remove_cvref_t<invoke_result_t<_Func, const _Err&>>;
680     static_assert(__is_std_expected<_Gp>::value, "The result of f(error()) must be a specialization of std::expected");
681     static_assert(is_same_v<typename _Gp::value_type, _Tp>,
682                   "The result of f(error()) must have the same value_type as this expected");
683     if (has_value()) {
684       return _Gp(in_place, value());
685     }
686     return std::invoke(std::forward<_Func>(__f), error());
687   }
688 
689   template <class _Func>
690     requires is_constructible_v<_Tp, _Tp&&>
691   _LIBCPP_HIDE_FROM_ABI constexpr auto or_else(_Func&& __f) && {
692     using _Gp = remove_cvref_t<invoke_result_t<_Func, _Err&&>>;
693     static_assert(
694         __is_std_expected<_Gp>::value, "The result of f(std::move(error())) must be a specialization of std::expected");
695     static_assert(is_same_v<typename _Gp::value_type, _Tp>,
696                   "The result of f(std::move(error())) must have the same value_type as this expected");
697     if (has_value()) {
698       return _Gp(in_place, std::move(value()));
699     }
700     return std::invoke(std::forward<_Func>(__f), std::move(error()));
701   }
702 
703   template <class _Func>
704     requires is_constructible_v<_Tp, const _Tp&&>
705   _LIBCPP_HIDE_FROM_ABI constexpr auto or_else(_Func&& __f) const&& {
706     using _Gp = remove_cvref_t<invoke_result_t<_Func, const _Err&&>>;
707     static_assert(
708         __is_std_expected<_Gp>::value, "The result of f(std::move(error())) must be a specialization of std::expected");
709     static_assert(is_same_v<typename _Gp::value_type, _Tp>,
710                   "The result of f(std::move(error())) must have the same value_type as this expected");
711     if (has_value()) {
712       return _Gp(in_place, std::move(value()));
713     }
714     return std::invoke(std::forward<_Func>(__f), std::move(error()));
715   }
716 
717   template <class _Func>
718     requires is_constructible_v<_Err, _Err&>
719   _LIBCPP_HIDE_FROM_ABI constexpr auto transform(_Func&& __f) & {
720     using _Up = remove_cv_t<invoke_result_t<_Func, _Tp&>>;
721     if (!has_value()) {
722       return expected<_Up, _Err>(unexpect, error());
723     }
724     if constexpr (!is_void_v<_Up>) {
725       return expected<_Up, _Err>(__expected_construct_in_place_from_invoke_tag{}, std::forward<_Func>(__f), value());
726     } else {
727       std::invoke(std::forward<_Func>(__f), value());
728       return expected<_Up, _Err>();
729     }
730   }
731 
732   template <class _Func>
733     requires is_constructible_v<_Err, const _Err&>
734   _LIBCPP_HIDE_FROM_ABI constexpr auto transform(_Func&& __f) const& {
735     using _Up = remove_cv_t<invoke_result_t<_Func, const _Tp&>>;
736     if (!has_value()) {
737       return expected<_Up, _Err>(unexpect, error());
738     }
739     if constexpr (!is_void_v<_Up>) {
740       return expected<_Up, _Err>(__expected_construct_in_place_from_invoke_tag{}, std::forward<_Func>(__f), value());
741     } else {
742       std::invoke(std::forward<_Func>(__f), value());
743       return expected<_Up, _Err>();
744     }
745   }
746 
747   template <class _Func>
748     requires is_constructible_v<_Err, _Err&&>
749   _LIBCPP_HIDE_FROM_ABI constexpr auto transform(_Func&& __f) && {
750     using _Up = remove_cv_t<invoke_result_t<_Func, _Tp&&>>;
751     if (!has_value()) {
752       return expected<_Up, _Err>(unexpect, std::move(error()));
753     }
754     if constexpr (!is_void_v<_Up>) {
755       return expected<_Up, _Err>(
756           __expected_construct_in_place_from_invoke_tag{}, std::forward<_Func>(__f), std::move(value()));
757     } else {
758       std::invoke(std::forward<_Func>(__f), std::move(value()));
759       return expected<_Up, _Err>();
760     }
761   }
762 
763   template <class _Func>
764     requires is_constructible_v<_Err, const _Err&&>
765   _LIBCPP_HIDE_FROM_ABI constexpr auto transform(_Func&& __f) const&& {
766     using _Up = remove_cv_t<invoke_result_t<_Func, const _Tp&&>>;
767     if (!has_value()) {
768       return expected<_Up, _Err>(unexpect, std::move(error()));
769     }
770     if constexpr (!is_void_v<_Up>) {
771       return expected<_Up, _Err>(
772           __expected_construct_in_place_from_invoke_tag{}, std::forward<_Func>(__f), std::move(value()));
773     } else {
774       std::invoke(std::forward<_Func>(__f), std::move(value()));
775       return expected<_Up, _Err>();
776     }
777   }
778 
779   template <class _Func>
780     requires is_constructible_v<_Tp, _Tp&>
781   _LIBCPP_HIDE_FROM_ABI constexpr auto transform_error(_Func&& __f) & {
782     using _Gp = remove_cv_t<invoke_result_t<_Func, _Err&>>;
783     static_assert(__valid_std_unexpected<_Gp>::value,
784                   "The result of f(error()) must be a valid template argument for unexpected");
785     if (has_value()) {
786       return expected<_Tp, _Gp>(in_place, value());
787     }
788     return expected<_Tp, _Gp>(__expected_construct_unexpected_from_invoke_tag{}, std::forward<_Func>(__f), error());
789   }
790 
791   template <class _Func>
792     requires is_constructible_v<_Tp, const _Tp&>
793   _LIBCPP_HIDE_FROM_ABI constexpr auto transform_error(_Func&& __f) const& {
794     using _Gp = remove_cv_t<invoke_result_t<_Func, const _Err&>>;
795     static_assert(__valid_std_unexpected<_Gp>::value,
796                   "The result of f(error()) must be a valid template argument for unexpected");
797     if (has_value()) {
798       return expected<_Tp, _Gp>(in_place, value());
799     }
800     return expected<_Tp, _Gp>(__expected_construct_unexpected_from_invoke_tag{}, std::forward<_Func>(__f), error());
801   }
802 
803   template <class _Func>
804     requires is_constructible_v<_Tp, _Tp&&>
805   _LIBCPP_HIDE_FROM_ABI constexpr auto transform_error(_Func&& __f) && {
806     using _Gp = remove_cv_t<invoke_result_t<_Func, _Err&&>>;
807     static_assert(__valid_std_unexpected<_Gp>::value,
808                   "The result of f(std::move(error())) must be a valid template argument for unexpected");
809     if (has_value()) {
810       return expected<_Tp, _Gp>(in_place, std::move(value()));
811     }
812     return expected<_Tp, _Gp>(
813         __expected_construct_unexpected_from_invoke_tag{}, std::forward<_Func>(__f), std::move(error()));
814   }
815 
816   template <class _Func>
817     requires is_constructible_v<_Tp, const _Tp&&>
818   _LIBCPP_HIDE_FROM_ABI constexpr auto transform_error(_Func&& __f) const&& {
819     using _Gp = remove_cv_t<invoke_result_t<_Func, const _Err&&>>;
820     static_assert(__valid_std_unexpected<_Gp>::value,
821                   "The result of f(std::move(error())) must be a valid template argument for unexpected");
822     if (has_value()) {
823       return expected<_Tp, _Gp>(in_place, std::move(value()));
824     }
825     return expected<_Tp, _Gp>(
826         __expected_construct_unexpected_from_invoke_tag{}, std::forward<_Func>(__f), std::move(error()));
827   }
828 
829   // [expected.object.eq], equality operators
830   template <class _T2, class _E2>
831     requires(!is_void_v<_T2>)
832   _LIBCPP_HIDE_FROM_ABI friend constexpr bool operator==(const expected& __x, const expected<_T2, _E2>& __y) {
833     if (__x.__has_val_ != __y.__has_val_) {
834       return false;
835     } else {
836       if (__x.__has_val_) {
837         return __x.__union_.__val_ == __y.__union_.__val_;
838       } else {
839         return __x.__union_.__unex_ == __y.__union_.__unex_;
840       }
841     }
842   }
843 
844   template <class _T2>
845   _LIBCPP_HIDE_FROM_ABI friend constexpr bool operator==(const expected& __x, const _T2& __v) {
846     return __x.__has_val_ && static_cast<bool>(__x.__union_.__val_ == __v);
847   }
848 
849   template <class _E2>
850   _LIBCPP_HIDE_FROM_ABI friend constexpr bool operator==(const expected& __x, const unexpected<_E2>& __e) {
851     return !__x.__has_val_ && static_cast<bool>(__x.__union_.__unex_ == __e.error());
852   }
853 
854 private:
855   template <class _ValueType, class _ErrorType>
856   union __union_t {
857     template <class... _Args>
858     _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(std::in_place_t, _Args&&... __args)
859         : __val_(std::forward<_Args>(__args)...) {}
860 
861     template <class... _Args>
862     _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(std::unexpect_t, _Args&&... __args)
863         : __unex_(std::forward<_Args>(__args)...) {}
864 
865     template <class _Func, class... _Args>
866     _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(
867         std::__expected_construct_in_place_from_invoke_tag, _Func&& __f, _Args&&... __args)
868         : __val_(std::invoke(std::forward<_Func>(__f), std::forward<_Args>(__args)...)) {}
869 
870     template <class _Func, class... _Args>
871     _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(
872         std::__expected_construct_unexpected_from_invoke_tag, _Func&& __f, _Args&&... __args)
873         : __unex_(std::invoke(std::forward<_Func>(__f), std::forward<_Args>(__args)...)) {}
874 
875     template <class _Union>
876     _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(bool __has_val, _Union&& __other) {
877       if (__has_val)
878         std::construct_at(std::addressof(__val_), std::forward<_Union>(__other).__val_);
879       else
880         std::construct_at(std::addressof(__unex_), std::forward<_Union>(__other).__unex_);
881     }
882 
883     _LIBCPP_HIDE_FROM_ABI constexpr ~__union_t()
884       requires(is_trivially_destructible_v<_ValueType> && is_trivially_destructible_v<_ErrorType>)
885     = default;
886 
887     // the expected's destructor handles this
888     _LIBCPP_HIDE_FROM_ABI constexpr ~__union_t() {}
889 
890     _ValueType __val_;
891     _ErrorType __unex_;
892   };
893 
894   // use named union because [[no_unique_address]] cannot be applied to an unnamed union,
895   // also guaranteed elision into a potentially-overlapping subobject is unsettled (and
896   // it's not clear that it's implementable, given that the function is allowed to clobber
897   // the tail padding) - see https://github.com/itanium-cxx-abi/cxx-abi/issues/107.
898   template <class _ValueType, class _ErrorType>
899     requires(is_trivially_move_constructible_v<_ValueType> && is_trivially_move_constructible_v<_ErrorType>)
900   union __union_t<_ValueType, _ErrorType> {
901     _LIBCPP_HIDE_FROM_ABI constexpr __union_t(const __union_t&) = default;
902     _LIBCPP_HIDE_FROM_ABI constexpr __union_t& operator=(const __union_t&) = default;
903 
904     template <class... _Args>
905     _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(std::in_place_t, _Args&&... __args)
906         : __val_(std::forward<_Args>(__args)...) {}
907 
908     template <class... _Args>
909     _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(std::unexpect_t, _Args&&... __args)
910         : __unex_(std::forward<_Args>(__args)...) {}
911 
912     template <class _Func, class... _Args>
913     _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(
914         std::__expected_construct_in_place_from_invoke_tag, _Func&& __f, _Args&&... __args)
915         : __val_(std::invoke(std::forward<_Func>(__f), std::forward<_Args>(__args)...)) {}
916 
917     template <class _Func, class... _Args>
918     _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(
919         std::__expected_construct_unexpected_from_invoke_tag, _Func&& __f, _Args&&... __args)
920         : __unex_(std::invoke(std::forward<_Func>(__f), std::forward<_Args>(__args)...)) {}
921 
922     template <class _Union>
923     _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(bool __has_val, _Union&& __other) {
924       if (__has_val)
925         std::construct_at(std::addressof(__val_), std::forward<_Union>(__other).__val_);
926       else
927         std::construct_at(std::addressof(__unex_), std::forward<_Union>(__other).__unex_);
928     }
929 
930     _LIBCPP_HIDE_FROM_ABI constexpr ~__union_t()
931       requires(is_trivially_destructible_v<_ValueType> && is_trivially_destructible_v<_ErrorType>)
932     = default;
933 
934     // the expected's destructor handles this
935     _LIBCPP_HIDE_FROM_ABI constexpr ~__union_t()
936       requires(!is_trivially_destructible_v<_ValueType> || !is_trivially_destructible_v<_ErrorType>)
937     {}
938 
939     _LIBCPP_NO_UNIQUE_ADDRESS _ValueType __val_;
940     _LIBCPP_NO_UNIQUE_ADDRESS _ErrorType __unex_;
941   };
942 
943   _LIBCPP_NO_UNIQUE_ADDRESS __union_t<_Tp, _Err> __union_;
944   bool __has_val_;
945 };
946 
947 template <class _Tp, class _Err>
948   requires is_void_v<_Tp>
949 class expected<_Tp, _Err> {
950   static_assert(__valid_std_unexpected<_Err>::value,
951                 "[expected.void.general] A program that instantiates expected<T, E> with a E that is not a "
952                 "valid argument for unexpected<E> is ill-formed");
953 
954   template <class, class>
955   friend class expected;
956 
957   template <class _Up, class _OtherErr, class _OtherErrQual>
958   using __can_convert =
959       _And< is_void<_Up>,
960             is_constructible<_Err, _OtherErrQual>,
961             _Not<is_constructible<unexpected<_Err>, expected<_Up, _OtherErr>&>>,
962             _Not<is_constructible<unexpected<_Err>, expected<_Up, _OtherErr>>>,
963             _Not<is_constructible<unexpected<_Err>, const expected<_Up, _OtherErr>&>>,
964             _Not<is_constructible<unexpected<_Err>, const expected<_Up, _OtherErr>>>>;
965 
966 public:
967   using value_type      = _Tp;
968   using error_type      = _Err;
969   using unexpected_type = unexpected<_Err>;
970 
971   template <class _Up>
972   using rebind = expected<_Up, error_type>;
973 
974   // [expected.void.ctor], constructors
975   _LIBCPP_HIDE_FROM_ABI constexpr expected() noexcept : __has_val_(true) {}
976 
977   _LIBCPP_HIDE_FROM_ABI constexpr expected(const expected&) = delete;
978 
979   _LIBCPP_HIDE_FROM_ABI constexpr expected(const expected&)
980     requires(is_copy_constructible_v<_Err> && is_trivially_copy_constructible_v<_Err>)
981   = default;
982 
983   _LIBCPP_HIDE_FROM_ABI constexpr expected(const expected& __rhs)
984     noexcept(is_nothrow_copy_constructible_v<_Err>) // strengthened
985     requires(is_copy_constructible_v<_Err> && !is_trivially_copy_constructible_v<_Err>)
986       : __union_(__rhs.__has_val_, __rhs.__union_), __has_val_(__rhs.__has_val_) {}
987 
988   _LIBCPP_HIDE_FROM_ABI constexpr expected(expected&&)
989     requires(is_move_constructible_v<_Err> && is_trivially_move_constructible_v<_Err>)
990   = default;
991 
992   _LIBCPP_HIDE_FROM_ABI constexpr expected(expected&& __rhs)
993     noexcept(is_nothrow_move_constructible_v<_Err>)
994     requires(is_move_constructible_v<_Err> && !is_trivially_move_constructible_v<_Err>)
995       : __union_(__rhs.__has_val_, std::move(__rhs.__union_)), __has_val_(__rhs.__has_val_) {}
996 
997   template <class _Up, class _OtherErr>
998     requires __can_convert<_Up, _OtherErr, const _OtherErr&>::value
999   _LIBCPP_HIDE_FROM_ABI constexpr explicit(!is_convertible_v<const _OtherErr&, _Err>)
1000   expected(const expected<_Up, _OtherErr>& __rhs)
1001     noexcept(is_nothrow_constructible_v<_Err, const _OtherErr&>) // strengthened
1002       : __union_(__rhs.__has_val_, __rhs.__union_), __has_val_(__rhs.__has_val_) {}
1003 
1004   template <class _Up, class _OtherErr>
1005     requires __can_convert<_Up, _OtherErr, _OtherErr>::value
1006   _LIBCPP_HIDE_FROM_ABI constexpr explicit(!is_convertible_v<_OtherErr, _Err>)
1007   expected(expected<_Up, _OtherErr>&& __rhs)
1008     noexcept(is_nothrow_constructible_v<_Err, _OtherErr>) // strengthened
1009       : __union_(__rhs.__has_val_, std::move(__rhs.__union_)), __has_val_(__rhs.__has_val_) {}
1010 
1011   template <class _OtherErr>
1012     requires is_constructible_v<_Err, const _OtherErr&>
1013   _LIBCPP_HIDE_FROM_ABI constexpr explicit(!is_convertible_v<const _OtherErr&, _Err>)
1014   expected(const unexpected<_OtherErr>& __unex)
1015     noexcept(is_nothrow_constructible_v<_Err, const _OtherErr&>) // strengthened
1016       : __union_(std::unexpect, __unex.error()), __has_val_(false) {}
1017 
1018   template <class _OtherErr>
1019     requires is_constructible_v<_Err, _OtherErr>
1020   _LIBCPP_HIDE_FROM_ABI constexpr explicit(!is_convertible_v<_OtherErr, _Err>)
1021   expected(unexpected<_OtherErr>&& __unex)
1022     noexcept(is_nothrow_constructible_v<_Err, _OtherErr>) // strengthened
1023       : __union_(std::unexpect, std::move(__unex.error())), __has_val_(false) {}
1024 
1025   _LIBCPP_HIDE_FROM_ABI constexpr explicit expected(in_place_t) noexcept : __has_val_(true) {}
1026 
1027   template <class... _Args>
1028     requires is_constructible_v<_Err, _Args...>
1029   _LIBCPP_HIDE_FROM_ABI constexpr explicit expected(unexpect_t, _Args&&... __args)
1030     noexcept(is_nothrow_constructible_v<_Err, _Args...>) // strengthened
1031       : __union_(std::unexpect, std::forward<_Args>(__args)...), __has_val_(false) {}
1032 
1033   template <class _Up, class... _Args>
1034     requires is_constructible_v< _Err, initializer_list<_Up>&, _Args... >
1035   _LIBCPP_HIDE_FROM_ABI constexpr explicit expected(unexpect_t, initializer_list<_Up> __il, _Args&&... __args)
1036     noexcept(is_nothrow_constructible_v<_Err, initializer_list<_Up>&, _Args...>) // strengthened
1037       : __union_(std::unexpect, __il, std::forward<_Args>(__args)...), __has_val_(false) {}
1038 
1039 private:
1040   template <class _Func>
1041   _LIBCPP_HIDE_FROM_ABI constexpr explicit expected(__expected_construct_in_place_from_invoke_tag, _Func&& __f)
1042       : __has_val_(true) {
1043     std::invoke(std::forward<_Func>(__f));
1044   }
1045 
1046   template <class _Func, class... _Args>
1047   _LIBCPP_HIDE_FROM_ABI constexpr explicit expected(
1048       __expected_construct_unexpected_from_invoke_tag __tag, _Func&& __f, _Args&&... __args)
1049       : __union_(__tag, std::forward<_Func>(__f), std::forward<_Args>(__args)...), __has_val_(false) {}
1050 
1051 public:
1052   // [expected.void.dtor], destructor
1053 
1054   _LIBCPP_HIDE_FROM_ABI constexpr ~expected()
1055     requires is_trivially_destructible_v<_Err>
1056   = default;
1057 
1058   _LIBCPP_HIDE_FROM_ABI constexpr ~expected()
1059     requires(!is_trivially_destructible_v<_Err>)
1060   {
1061     if (!__has_val_) {
1062       std::destroy_at(std::addressof(__union_.__unex_));
1063     }
1064   }
1065 
1066   // [expected.void.assign], assignment
1067 
1068   _LIBCPP_HIDE_FROM_ABI constexpr expected& operator=(const expected&) = delete;
1069 
1070   _LIBCPP_HIDE_FROM_ABI constexpr expected& operator=(const expected& __rhs)
1071     noexcept(is_nothrow_copy_assignable_v<_Err> && is_nothrow_copy_constructible_v<_Err>) // strengthened
1072     requires(is_copy_assignable_v<_Err> && is_copy_constructible_v<_Err>)
1073   {
1074     if (__has_val_) {
1075       if (!__rhs.__has_val_) {
1076         std::construct_at(std::addressof(__union_.__unex_), __rhs.__union_.__unex_);
1077         __has_val_ = false;
1078       }
1079     } else {
1080       if (__rhs.__has_val_) {
1081         std::destroy_at(std::addressof(__union_.__unex_));
1082         __has_val_ = true;
1083       } else {
1084         __union_.__unex_ = __rhs.__union_.__unex_;
1085       }
1086     }
1087     return *this;
1088   }
1089 
1090   _LIBCPP_HIDE_FROM_ABI constexpr expected& operator=(expected&&) = delete;
1091 
1092   _LIBCPP_HIDE_FROM_ABI constexpr expected& operator=(expected&& __rhs)
1093     noexcept(is_nothrow_move_assignable_v<_Err> &&
1094              is_nothrow_move_constructible_v<_Err>)
1095     requires(is_move_assignable_v<_Err> &&
1096              is_move_constructible_v<_Err>)
1097   {
1098     if (__has_val_) {
1099       if (!__rhs.__has_val_) {
1100         std::construct_at(std::addressof(__union_.__unex_), std::move(__rhs.__union_.__unex_));
1101         __has_val_ = false;
1102       }
1103     } else {
1104       if (__rhs.__has_val_) {
1105         std::destroy_at(std::addressof(__union_.__unex_));
1106         __has_val_ = true;
1107       } else {
1108         __union_.__unex_ = std::move(__rhs.__union_.__unex_);
1109       }
1110     }
1111     return *this;
1112   }
1113 
1114   template <class _OtherErr>
1115     requires(is_constructible_v<_Err, const _OtherErr&> && is_assignable_v<_Err&, const _OtherErr&>)
1116   _LIBCPP_HIDE_FROM_ABI constexpr expected& operator=(const unexpected<_OtherErr>& __un) {
1117     if (__has_val_) {
1118       std::construct_at(std::addressof(__union_.__unex_), __un.error());
1119       __has_val_ = false;
1120     } else {
1121       __union_.__unex_ = __un.error();
1122     }
1123     return *this;
1124   }
1125 
1126   template <class _OtherErr>
1127     requires(is_constructible_v<_Err, _OtherErr> && is_assignable_v<_Err&, _OtherErr>)
1128   _LIBCPP_HIDE_FROM_ABI constexpr expected& operator=(unexpected<_OtherErr>&& __un) {
1129     if (__has_val_) {
1130       std::construct_at(std::addressof(__union_.__unex_), std::move(__un.error()));
1131       __has_val_ = false;
1132     } else {
1133       __union_.__unex_ = std::move(__un.error());
1134     }
1135     return *this;
1136   }
1137 
1138   _LIBCPP_HIDE_FROM_ABI constexpr void emplace() noexcept {
1139     if (!__has_val_) {
1140       std::destroy_at(std::addressof(__union_.__unex_));
1141       __has_val_ = true;
1142     }
1143   }
1144 
1145   // [expected.void.swap], swap
1146   _LIBCPP_HIDE_FROM_ABI constexpr void swap(expected& __rhs)
1147     noexcept(is_nothrow_move_constructible_v<_Err> && is_nothrow_swappable_v<_Err>)
1148     requires(is_swappable_v<_Err> && is_move_constructible_v<_Err>)
1149   {
1150     auto __swap_val_unex_impl = [&](expected& __with_val, expected& __with_err) {
1151       std::construct_at(std::addressof(__with_val.__union_.__unex_), std::move(__with_err.__union_.__unex_));
1152       std::destroy_at(std::addressof(__with_err.__union_.__unex_));
1153       __with_val.__has_val_ = false;
1154       __with_err.__has_val_ = true;
1155     };
1156 
1157     if (__has_val_) {
1158       if (!__rhs.__has_val_) {
1159         __swap_val_unex_impl(*this, __rhs);
1160       }
1161     } else {
1162       if (__rhs.__has_val_) {
1163         __swap_val_unex_impl(__rhs, *this);
1164       } else {
1165         using std::swap;
1166         swap(__union_.__unex_, __rhs.__union_.__unex_);
1167       }
1168     }
1169   }
1170 
1171   _LIBCPP_HIDE_FROM_ABI friend constexpr void swap(expected& __x, expected& __y)
1172     noexcept(noexcept(__x.swap(__y)))
1173     requires requires { __x.swap(__y); }
1174   {
1175     __x.swap(__y);
1176   }
1177 
1178   // [expected.void.obs], observers
1179   _LIBCPP_HIDE_FROM_ABI constexpr explicit operator bool() const noexcept { return __has_val_; }
1180 
1181   _LIBCPP_HIDE_FROM_ABI constexpr bool has_value() const noexcept { return __has_val_; }
1182 
1183   _LIBCPP_HIDE_FROM_ABI constexpr void operator*() const noexcept {
1184     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(__has_val_, "expected::operator* requires the expected to contain a value");
1185   }
1186 
1187   _LIBCPP_HIDE_FROM_ABI constexpr void value() const& {
1188     if (!__has_val_) {
1189       std::__throw_bad_expected_access<_Err>(__union_.__unex_);
1190     }
1191   }
1192 
1193   _LIBCPP_HIDE_FROM_ABI constexpr void value() && {
1194     if (!__has_val_) {
1195       std::__throw_bad_expected_access<_Err>(std::move(__union_.__unex_));
1196     }
1197   }
1198 
1199   _LIBCPP_HIDE_FROM_ABI constexpr const _Err& error() const& noexcept {
1200     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(!__has_val_, "expected::error requires the expected to contain an error");
1201     return __union_.__unex_;
1202   }
1203 
1204   _LIBCPP_HIDE_FROM_ABI constexpr _Err& error() & noexcept {
1205     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(!__has_val_, "expected::error requires the expected to contain an error");
1206     return __union_.__unex_;
1207   }
1208 
1209   _LIBCPP_HIDE_FROM_ABI constexpr const _Err&& error() const&& noexcept {
1210     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(!__has_val_, "expected::error requires the expected to contain an error");
1211     return std::move(__union_.__unex_);
1212   }
1213 
1214   _LIBCPP_HIDE_FROM_ABI constexpr _Err&& error() && noexcept {
1215     _LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(!__has_val_, "expected::error requires the expected to contain an error");
1216     return std::move(__union_.__unex_);
1217   }
1218 
1219   template <class _Up = _Err>
1220   _LIBCPP_HIDE_FROM_ABI constexpr _Err error_or(_Up&& __error) const& {
1221     static_assert(is_copy_constructible_v<_Err>, "error_type has to be copy constructible");
1222     static_assert(is_convertible_v<_Up, _Err>, "argument has to be convertible to error_type");
1223     if (has_value()) {
1224       return std::forward<_Up>(__error);
1225     }
1226     return error();
1227   }
1228 
1229   template <class _Up = _Err>
1230   _LIBCPP_HIDE_FROM_ABI constexpr _Err error_or(_Up&& __error) && {
1231     static_assert(is_move_constructible_v<_Err>, "error_type has to be move constructible");
1232     static_assert(is_convertible_v<_Up, _Err>, "argument has to be convertible to error_type");
1233     if (has_value()) {
1234       return std::forward<_Up>(__error);
1235     }
1236     return std::move(error());
1237   }
1238 
1239   // [expected.void.monadic], monadic
1240   template <class _Func>
1241     requires is_constructible_v<_Err, _Err&>
1242   _LIBCPP_HIDE_FROM_ABI constexpr auto and_then(_Func&& __f) & {
1243     using _Up = remove_cvref_t<invoke_result_t<_Func>>;
1244     static_assert(__is_std_expected<_Up>::value, "The result of f() must be a specialization of std::expected");
1245     static_assert(
1246         is_same_v<typename _Up::error_type, _Err>, "The result of f() must have the same error_type as this expected");
1247     if (has_value()) {
1248       return std::invoke(std::forward<_Func>(__f));
1249     }
1250     return _Up(unexpect, error());
1251   }
1252 
1253   template <class _Func>
1254     requires is_constructible_v<_Err, const _Err&>
1255   _LIBCPP_HIDE_FROM_ABI constexpr auto and_then(_Func&& __f) const& {
1256     using _Up = remove_cvref_t<invoke_result_t<_Func>>;
1257     static_assert(__is_std_expected<_Up>::value, "The result of f() must be a specialization of std::expected");
1258     static_assert(
1259         is_same_v<typename _Up::error_type, _Err>, "The result of f() must have the same error_type as this expected");
1260     if (has_value()) {
1261       return std::invoke(std::forward<_Func>(__f));
1262     }
1263     return _Up(unexpect, error());
1264   }
1265 
1266   template <class _Func>
1267     requires is_constructible_v<_Err, _Err&&>
1268   _LIBCPP_HIDE_FROM_ABI constexpr auto and_then(_Func&& __f) && {
1269     using _Up = remove_cvref_t<invoke_result_t<_Func>>;
1270     static_assert(__is_std_expected<_Up>::value, "The result of f() must be a specialization of std::expected");
1271     static_assert(
1272         is_same_v<typename _Up::error_type, _Err>, "The result of f() must have the same error_type as this expected");
1273     if (has_value()) {
1274       return std::invoke(std::forward<_Func>(__f));
1275     }
1276     return _Up(unexpect, std::move(error()));
1277   }
1278 
1279   template <class _Func>
1280     requires is_constructible_v<_Err, const _Err&&>
1281   _LIBCPP_HIDE_FROM_ABI constexpr auto and_then(_Func&& __f) const&& {
1282     using _Up = remove_cvref_t<invoke_result_t<_Func>>;
1283     static_assert(__is_std_expected<_Up>::value, "The result of f() must be a specialization of std::expected");
1284     static_assert(
1285         is_same_v<typename _Up::error_type, _Err>, "The result of f() must have the same error_type as this expected");
1286     if (has_value()) {
1287       return std::invoke(std::forward<_Func>(__f));
1288     }
1289     return _Up(unexpect, std::move(error()));
1290   }
1291 
1292   template <class _Func>
1293   _LIBCPP_HIDE_FROM_ABI constexpr auto or_else(_Func&& __f) & {
1294     using _Gp = remove_cvref_t<invoke_result_t<_Func, _Err&>>;
1295     static_assert(__is_std_expected<_Gp>::value, "The result of f(error()) must be a specialization of std::expected");
1296     static_assert(is_same_v<typename _Gp::value_type, _Tp>,
1297                   "The result of f(error()) must have the same value_type as this expected");
1298     if (has_value()) {
1299       return _Gp();
1300     }
1301     return std::invoke(std::forward<_Func>(__f), error());
1302   }
1303 
1304   template <class _Func>
1305   _LIBCPP_HIDE_FROM_ABI constexpr auto or_else(_Func&& __f) const& {
1306     using _Gp = remove_cvref_t<invoke_result_t<_Func, const _Err&>>;
1307     static_assert(__is_std_expected<_Gp>::value, "The result of f(error()) must be a specialization of std::expected");
1308     static_assert(is_same_v<typename _Gp::value_type, _Tp>,
1309                   "The result of f(error()) must have the same value_type as this expected");
1310     if (has_value()) {
1311       return _Gp();
1312     }
1313     return std::invoke(std::forward<_Func>(__f), error());
1314   }
1315 
1316   template <class _Func>
1317   _LIBCPP_HIDE_FROM_ABI constexpr auto or_else(_Func&& __f) && {
1318     using _Gp = remove_cvref_t<invoke_result_t<_Func, _Err&&>>;
1319     static_assert(__is_std_expected<_Gp>::value,
1320                   "The result of f(std::move(error())) must be a specialization of std::expected");
1321     static_assert(is_same_v<typename _Gp::value_type, _Tp>,
1322                   "The result of f(std::move(error())) must have the same value_type as this expected");
1323     if (has_value()) {
1324       return _Gp();
1325     }
1326     return std::invoke(std::forward<_Func>(__f), std::move(error()));
1327   }
1328 
1329   template <class _Func>
1330   _LIBCPP_HIDE_FROM_ABI constexpr auto or_else(_Func&& __f) const&& {
1331     using _Gp = remove_cvref_t<invoke_result_t<_Func, const _Err&&>>;
1332     static_assert(__is_std_expected<_Gp>::value,
1333                   "The result of f(std::move(error())) must be a specialization of std::expected");
1334     static_assert(is_same_v<typename _Gp::value_type, _Tp>,
1335                   "The result of f(std::move(error())) must have the same value_type as this expected");
1336     if (has_value()) {
1337       return _Gp();
1338     }
1339     return std::invoke(std::forward<_Func>(__f), std::move(error()));
1340   }
1341 
1342   template <class _Func>
1343     requires is_constructible_v<_Err, _Err&>
1344   _LIBCPP_HIDE_FROM_ABI constexpr auto transform(_Func&& __f) & {
1345     using _Up = remove_cv_t<invoke_result_t<_Func>>;
1346     if (!has_value()) {
1347       return expected<_Up, _Err>(unexpect, error());
1348     }
1349     if constexpr (!is_void_v<_Up>) {
1350       return expected<_Up, _Err>(__expected_construct_in_place_from_invoke_tag{}, std::forward<_Func>(__f));
1351     } else {
1352       std::invoke(std::forward<_Func>(__f));
1353       return expected<_Up, _Err>();
1354     }
1355   }
1356 
1357   template <class _Func>
1358     requires is_constructible_v<_Err, const _Err&>
1359   _LIBCPP_HIDE_FROM_ABI constexpr auto transform(_Func&& __f) const& {
1360     using _Up = remove_cv_t<invoke_result_t<_Func>>;
1361     if (!has_value()) {
1362       return expected<_Up, _Err>(unexpect, error());
1363     }
1364     if constexpr (!is_void_v<_Up>) {
1365       return expected<_Up, _Err>(__expected_construct_in_place_from_invoke_tag{}, std::forward<_Func>(__f));
1366     } else {
1367       std::invoke(std::forward<_Func>(__f));
1368       return expected<_Up, _Err>();
1369     }
1370   }
1371 
1372   template <class _Func>
1373     requires is_constructible_v<_Err, _Err&&>
1374   _LIBCPP_HIDE_FROM_ABI constexpr auto transform(_Func&& __f) && {
1375     using _Up = remove_cv_t<invoke_result_t<_Func>>;
1376     if (!has_value()) {
1377       return expected<_Up, _Err>(unexpect, std::move(error()));
1378     }
1379     if constexpr (!is_void_v<_Up>) {
1380       return expected<_Up, _Err>(__expected_construct_in_place_from_invoke_tag{}, std::forward<_Func>(__f));
1381     } else {
1382       std::invoke(std::forward<_Func>(__f));
1383       return expected<_Up, _Err>();
1384     }
1385   }
1386 
1387   template <class _Func>
1388     requires is_constructible_v<_Err, const _Err&&>
1389   _LIBCPP_HIDE_FROM_ABI constexpr auto transform(_Func&& __f) const&& {
1390     using _Up = remove_cv_t<invoke_result_t<_Func>>;
1391     if (!has_value()) {
1392       return expected<_Up, _Err>(unexpect, std::move(error()));
1393     }
1394     if constexpr (!is_void_v<_Up>) {
1395       return expected<_Up, _Err>(__expected_construct_in_place_from_invoke_tag{}, std::forward<_Func>(__f));
1396     } else {
1397       std::invoke(std::forward<_Func>(__f));
1398       return expected<_Up, _Err>();
1399     }
1400   }
1401 
1402   template <class _Func>
1403   _LIBCPP_HIDE_FROM_ABI constexpr auto transform_error(_Func&& __f) & {
1404     using _Gp = remove_cv_t<invoke_result_t<_Func, _Err&>>;
1405     static_assert(__valid_std_unexpected<_Gp>::value,
1406                   "The result of f(error()) must be a valid template argument for unexpected");
1407     if (has_value()) {
1408       return expected<_Tp, _Gp>();
1409     }
1410     return expected<_Tp, _Gp>(__expected_construct_unexpected_from_invoke_tag{}, std::forward<_Func>(__f), error());
1411   }
1412 
1413   template <class _Func>
1414   _LIBCPP_HIDE_FROM_ABI constexpr auto transform_error(_Func&& __f) const& {
1415     using _Gp = remove_cv_t<invoke_result_t<_Func, const _Err&>>;
1416     static_assert(__valid_std_unexpected<_Gp>::value,
1417                   "The result of f(error()) must be a valid template argument for unexpected");
1418     if (has_value()) {
1419       return expected<_Tp, _Gp>();
1420     }
1421     return expected<_Tp, _Gp>(__expected_construct_unexpected_from_invoke_tag{}, std::forward<_Func>(__f), error());
1422   }
1423 
1424   template <class _Func>
1425   _LIBCPP_HIDE_FROM_ABI constexpr auto transform_error(_Func&& __f) && {
1426     using _Gp = remove_cv_t<invoke_result_t<_Func, _Err&&>>;
1427     static_assert(__valid_std_unexpected<_Gp>::value,
1428                   "The result of f(std::move(error())) must be a valid template argument for unexpected");
1429     if (has_value()) {
1430       return expected<_Tp, _Gp>();
1431     }
1432     return expected<_Tp, _Gp>(
1433         __expected_construct_unexpected_from_invoke_tag{}, std::forward<_Func>(__f), std::move(error()));
1434   }
1435 
1436   template <class _Func>
1437   _LIBCPP_HIDE_FROM_ABI constexpr auto transform_error(_Func&& __f) const&& {
1438     using _Gp = remove_cv_t<invoke_result_t<_Func, const _Err&&>>;
1439     static_assert(__valid_std_unexpected<_Gp>::value,
1440                   "The result of f(std::move(error())) must be a valid template argument for unexpected");
1441     if (has_value()) {
1442       return expected<_Tp, _Gp>();
1443     }
1444     return expected<_Tp, _Gp>(
1445         __expected_construct_unexpected_from_invoke_tag{}, std::forward<_Func>(__f), std::move(error()));
1446   }
1447 
1448   // [expected.void.eq], equality operators
1449   template <class _T2, class _E2>
1450     requires is_void_v<_T2>
1451   _LIBCPP_HIDE_FROM_ABI friend constexpr bool operator==(const expected& __x, const expected<_T2, _E2>& __y) {
1452     if (__x.__has_val_ != __y.__has_val_) {
1453       return false;
1454     } else {
1455       return __x.__has_val_ || static_cast<bool>(__x.__union_.__unex_ == __y.__union_.__unex_);
1456     }
1457   }
1458 
1459   template <class _E2>
1460   _LIBCPP_HIDE_FROM_ABI friend constexpr bool operator==(const expected& __x, const unexpected<_E2>& __y) {
1461     return !__x.__has_val_ && static_cast<bool>(__x.__union_.__unex_ == __y.error());
1462   }
1463 
1464 private:
1465   struct __empty_t {};
1466 
1467   template <class _ErrorType>
1468   union __union_t {
1469     _LIBCPP_HIDE_FROM_ABI constexpr __union_t() : __empty_() {}
1470 
1471     template <class... _Args>
1472     _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(std::unexpect_t, _Args&&... __args)
1473         : __unex_(std::forward<_Args>(__args)...) {}
1474 
1475     template <class _Func, class... _Args>
1476     _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(
1477         __expected_construct_unexpected_from_invoke_tag, _Func&& __f, _Args&&... __args)
1478         : __unex_(std::invoke(std::forward<_Func>(__f), std::forward<_Args>(__args)...)) {}
1479 
1480     template <class _Union>
1481     _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(bool __has_val, _Union&& __other) {
1482       if (__has_val)
1483         std::construct_at(std::addressof(__empty_));
1484       else
1485         std::construct_at(std::addressof(__unex_), std::forward<_Union>(__other).__unex_);
1486     }
1487 
1488     _LIBCPP_HIDE_FROM_ABI constexpr ~__union_t()
1489       requires(is_trivially_destructible_v<_ErrorType>)
1490     = default;
1491 
1492     // the expected's destructor handles this
1493     _LIBCPP_HIDE_FROM_ABI constexpr ~__union_t() {}
1494 
1495     __empty_t __empty_;
1496     _ErrorType __unex_;
1497   };
1498 
1499   // use named union because [[no_unique_address]] cannot be applied to an unnamed union,
1500   // also guaranteed elision into a potentially-overlapping subobject is unsettled (and
1501   // it's not clear that it's implementable, given that the function is allowed to clobber
1502   // the tail padding) - see https://github.com/itanium-cxx-abi/cxx-abi/issues/107.
1503   template <class _ErrorType>
1504     requires is_trivially_move_constructible_v<_ErrorType>
1505   union __union_t<_ErrorType> {
1506     _LIBCPP_HIDE_FROM_ABI constexpr __union_t() : __empty_() {}
1507 
1508     template <class... _Args>
1509     _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(std::unexpect_t, _Args&&... __args)
1510         : __unex_(std::forward<_Args>(__args)...) {}
1511 
1512     template <class _Func, class... _Args>
1513     _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(
1514         __expected_construct_unexpected_from_invoke_tag, _Func&& __f, _Args&&... __args)
1515         : __unex_(std::invoke(std::forward<_Func>(__f), std::forward<_Args>(__args)...)) {}
1516 
1517     template <class _Union>
1518     _LIBCPP_HIDE_FROM_ABI constexpr explicit __union_t(bool __has_val, _Union&& __other) {
1519       if (__has_val)
1520         std::construct_at(std::addressof(__empty_));
1521       else
1522         std::construct_at(std::addressof(__unex_), std::forward<_Union>(__other).__unex_);
1523     }
1524 
1525     _LIBCPP_HIDE_FROM_ABI constexpr ~__union_t()
1526       requires(is_trivially_destructible_v<_ErrorType>)
1527     = default;
1528 
1529     // the expected's destructor handles this
1530     _LIBCPP_HIDE_FROM_ABI constexpr ~__union_t()
1531       requires(!is_trivially_destructible_v<_ErrorType>)
1532     {}
1533 
1534     _LIBCPP_NO_UNIQUE_ADDRESS __empty_t __empty_;
1535     _LIBCPP_NO_UNIQUE_ADDRESS _ErrorType __unex_;
1536   };
1537 
1538   _LIBCPP_NO_UNIQUE_ADDRESS __union_t<_Err> __union_;
1539   bool __has_val_;
1540 };
1541 
1542 _LIBCPP_END_NAMESPACE_STD
1543 
1544 #endif // _LIBCPP_STD_VER >= 23
1545 
1546 _LIBCPP_POP_MACROS
1547 
1548 #endif // _LIBCPP___EXPECTED_EXPECTED_H
1549